Metal containers including aluminum cans are usually coated before use to protect the interior and exterior surfaces of the container from deterioration. It is conventional to coat the interior surfaces of the container with a coating composition that has a higher film weight than that of the coating composition applied to the exterior surfaces of the container so that the interior coating will be thicker.
The thicker interior coating protects the metal surfaces of the container from the contents of the container and also protects the contents of the container from reaction with the metal. The thinner exterior coating protects the container from ambient conditions by inhibiting the corrosion of the metal container. The exterior coating is also applied to improve the handling characteristics of the container and for aesthetic reasons.
The coating compositions have been applied by spray, roll or immersion methods or by using conventional electrocoating techniques. Electrocoating has many advantages in that the electrocoating process minimizes solvent emissions, maximizes coating utilization and provides uniform and consistent films.
As used herein, the term "electrocoating" includes the electrodeposition of resinous coating compositions on electrically conductive surfaces from either anodic or cathodic electrocoating material mediums. When a metal surface is contacted with a coating composition and an electrical potential is applied between the metal surface and an oppositely charged electrode, a layer of a coating composition may be electrodeposited on the metal surface.
A process and an apparatus for simultaneously electrocoating the interior and exterior surfaces of metal containers with different coating compositions at a high speed is disclosed in U.S. Pat. No. 4,400,251 to Heffner et al., which patent is assigned to the Aluminum Company of America.
Conventional epoxy/phenolic coating compositions provide hard, flexible and solvent resistant coatings which are desired for sanitary can coatings, but such coating compositions are relatively slow curing. As a result, production times are decreased relative to a rapidly curing composition. That is a definite disadvantage when such coating compositions are used with a high speed electrocoating apparatus of the type described in the above-referenced patent.
A need exists for an electrocoatable coating composition that forms a cured coating with the desired properties, but which possesses a relatively short cure time for use in the coating of metal surfaces including sanitary cans, and in particular, aluminum cans.